(1) Field of the Invention
This invention relates to X-ray CT apparatus for performing X-ray computerized tomography (X-ray CT) with contrast media injected to patients, in which a CT for acquiring images for a diagnostic purpose (hereinafter called "production CT") is preceded by a pre-CT for a monitoring purpose which is carried out with low doses in order to determine the timing of starting the production CT. More particularly, the invention relates to a technique for carrying out the pre-CT in an appropriate radiographic mode.
(2) Description of the Related Art
An X-ray CT apparatus (X-ray computerized tomographic apparatus) used in a hospital or the like, as shown in FIG. 1, includes an X-ray tube 51 and a multi-channel X-ray detector 52 opposed to each other across a patient M placed on a top board 50. While the X-ray tube 51 and X-ray detector 52 are revolved about the body axis of patient M, a fan beam FB is emitted from the X-ray tube 51. Based on X-ray detection data outputted from the X-ray detector 52, an image reconstruction process is carried out to acquire computed tomographic images (CT images).
The conventional apparatus may be used to perform an X-ray CT of an internal organ (e.g. the liver) of a patient with a contrast medium applied thereto. This is done in order to check presence and position of a tumor by fixation (i.e. coloring) of the contrast medium in a region of concern in the CT images acquired. However, in performing an X-ray CT with a contrast medium, it is necessary to start a radiographic process (production CT) when the contrast medium has fully spread to the region of concern such as the liver.
Thus, the conventional X-ray CT apparatus allows a pre-CT to be carried out with low doses for monitoring purposes before a production CT in order to check whether the contrast medium has spread to the region of concern. The production CT is started after confirming by the CT images acquired from the pre-CT that the contrast medium has fully spread to the region of concern.
Further, it is conventional practice to perform the pre-CT in a combination of intermittent and continuous radiographic processes in which an intermittent radiography is executed for a predetermined period, and then a continuous radiography is executed. Specifically, as shown in FIG. 2, spreading conditions of the contrast medium are checked from time to time by observing CT images acquired from the intermittent radiography at an initial stage where the contrast medium has not spread to the full. When the contrast medium is thought to have spread to some extent, spreading conditions of the contrast medium are checked continuously by means of CT images acquired from the continuous radiography. Exposure dose is reduced for the intermittent radiography.
However, the conventional X-ray CT apparatus has a drawback of sometimes failing to determine the timing of starting the production CT correctly. This is because the pre-CT performed in the combination of intermittent and continuous radiographic processes can be ill suited for checking the spreading conditions of the contrast medium. On such an occasion, it is impossible to determine the timing of starting the production CT correctly.